THE MCCOLLOUGH EFFECT ACROSS THE MENSTRUAL-CYCLE

The McCollough effect (ME) has been shown to be sensitive to cholinerg ic agents, being strengthened by hyoscine (antagonist) and weakened by physostigmine (agonist), and possibly to more generalized changes in CNS arousal. We therefore expected the ME to be sensitive to hormonal changes during the menstrual cycle, being strongest in the postovulato ry phases when arousal is low. In two experiments we found a highly si gnificant effect of menstrual phase for the normally cycling women, bu t not for oral contraceptive users: ME strength gradually increased ac ross the cycle, reaching a premenstrual peak. These findings may be ex plained in terms of hormonally mediated changes in arousal across the menstrual cycle.